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Spacecraft looks for its 3,000th comet

Elizabeth Howell, News Writer
Apr 25, 2015, 4:11 UTC

Sen—Less than two years ago, the world watched entranced as a comet broke up into pieces in front of the watchful eye of a spacecraft.

Called Comet ISON, the celestial visitor was supposed to put on an excellent show for stargazers around Christmas 2013. Instead, the comet went out with a bang during American Thanksgiving in November, flaring up before dissolving.

The dramatic end played out in front of a spacecraft called SOHO (Solar and Heliospheric Observatory (SOHO), jointly operated by NASA and the European Space Agency (ESA). As its name implies, SOHO is supposed to provide information about the Sun and its atmosphere. But it also turns out to be an excellent comet-hunter.

And now, the public has an additional chance to participate—to guess when SOHO finds its 3,000th comet, which you can try out on this page. To date, SOHO has found roughly 2,890 comets; the contest will stop taking entries whenever the observatory has found 2,950.


SOHO's primary mission is to observe the Sun, but it is also an excellent comet-hunter. Image credit: NASA/ESA

While the comet discoveries were not a complete surprise, after SOHO's launch scientists were impressed at how well the CCD detectors performed, NASA solar physicist Joseph Gurman told Sen.

"We first noted it in the faintness of some coronal mass ejections we were able to see early on," he wrote in an email. "It was less of a surprise when we realized how many more comets we’d be seeing than we’d been able to see with previous generations of spaceborne coronagraphs, built when the state of the art in detectors offered much lower dynamic range."

Now the question is whether SOHO will find its 3,000th comet before the 20th anniversary of its launch, which took place on Dec. 2, 1995. More than 90 per cent of sungrazing comets have been found by amateurs going through SOHO images on the Internet.

"Simply put—they're absolutely critical," said Karl Battams, an astrophysicist at the Naval Research Laboratory who has operated the NASA-funded Sungrazing Comets project since 2003, in an email to Sen.

"Without their enthusiasm and dedication, a vast reservoir of science would be unrealized."


Artist's impression of the SOHO spacecraft. Image credit: ESA

While SOHO's observations of ISON are still fresh in the public's memory, Gurman pointed out that there have been many beautiful examples that passed by the spacecraft's cameras. This includes twin sungrazers in 1998 and Comet 96/P Machholz, which also caught Gurman's attention because the latter passed by during a spectacular coronal mass ejection from the Sun.

"The LASCO coronagraph is able to look at the region of space that's extremely close to the Sun, and invisible to essentially every other observatory (terrestrial or celestial)," Battams added. "By luck, it turns out there are a couple of well-populated groups of comets that fly through that region, and hence SOHO finds so many."

Battams said it has been a privilege to helm the sungrazing project for so long. And the world certainly looks forward to what comets SOHO sees next.